The present invention relates to an apparatus for measuring a flow rate of a fluid such as a gas.
Various apparatuses are known which are employed for measuring the flow rate of a fluid. The present applicant has made a proposal in the specification of U.S. Pat. No. 4,729,244 concerning an apparatus for measuring a flow rate employing a cantilever-shaped pressure-receiving plate. This apparatus is characterized by having, in addition to a structure for measuring the flow rate of a fluid by means of the cantilever-shaped pressure-receiving plate, a variable orifice structure formed by a by-pass passage provided in the flow passage of the fluid and a cantilever-shaped valve provided in the by-pass passage. In this apparatus, the amount of fluid flowing through the by-pass passage is controlled in accordance with the flow rate of the fluid, thereby enabling the flow rate to be measured over a wide range covering low flow-rate regions and high flow-rate regions; further, the measurement range can be changed simply by varying the diameter of the by-pass passage.
The flow rate measuring apparatus previously proposed by the applicant, however, encounters the following disadvantageous phenomenon. Because of the variable orifice structure in which a cantilever-shaped valve is specially provided in the by-pass passage, if the fluid to be measured flows at certain flow rates within the wide range covering high as well as low flow-rate regions, the fluid flows without forming a laminar flow. In such cases, erratic movement or fluctuation of the fluid may cause the valve to vibrate to a not negligible extent. The vibration of the valve may, in turn, amplify the fluctuation of the fluid, and, if the resultant movement of the fluid is transmitted to the cantilever-shaped pressure-receiving plate provided for measurement, the result is that the apparatus will fail to achieve stable measurement.